Mixed matrix membranes have proven to be effective in separating gas components contained within a gaseous mixture. The mixed matrix membranes typically contain molecular sieves which are embedded within polymeric organic materials. Mixed matrix membranes exhibit the unusual effect that the selectivity of the combined molecular sieves and organic polymer are higher than that of the organic polymer alone.
An example of such a mixed matrix membrane is found in U.S. Pat. No. 5,127,925 to Kulprathipanji et al. Another exemplary patent is U.S. Pat. No. 4,925,459 to Rojey et al. which describes the use of molecular sieves supported by an organic polymer to create a membrane which is useful for the separation of gas components. In both patents, membranes utilize zeolites as a molecular sieve. Zeolites are silica containing molecular sieves which have a particularly highly ordered crystalline structure often with desirable pore sizes and shapes conducive for fluid separations.
An example of the preparation of a zeolite, i.e., SSZ-13, is taught in U.S. Pat. No. 4,544,538 to Zones. Another example of preparation of a zeolite, SSZ-62, is described in U.S. patent application Ser. No. 2003/0069449 to Zones et al. The disclosures found in each of these patent documents are hereby incorporated by reference in their entireties.
The manufacture of zeolites used in mixed matrix membranes may include the step of lowering the concentration of alkali metals in the zeolite by converting the zeolite to a hydrogen form. This is conventionally done by ion exchange, generally with ammonium cations. After ion-exchange, the zeolite is calcined to decompose the ammonium cations, thereby converting the zeolite from an ammonium form to the hydrogen form.
While this method of treating zeolite particles prior to their incorporation into an organic polymer may benefit membrane selectivity and/or permeability to a degree, there is a need to discover improved zeolites and methods of treating those zeolites to achieve even better separation performance. While improved performance could also be achieved by increasing the zeolite content in a membrane, technical difficulties in membrane preparation (e.g., fiber spinning) and membrane strength can limit the upper percentage of zeolites that can be added. Accordingly, finding a way to get more effective use from a given content of zeolite would have distinct advantages. Higher selectivity will mean less loss of potentially valuable retentate to a permeate stream of fluids being separated. Higher permeability will reduce the required membrane area, thereby reducing investment cost.
In converting zeolite to the hydrogen form, it has been found that if ammonium cations are not completely removed, the residual cations can partially restrict diffusion of a gas, e.g., CO2, through pores in the zeolite, reducing membrane permeability and selectivity. Second, complete removal of the ammonium cations is difficult, requiring calcination at temperatures above 400° C., generally above 450° C. or even 500° C. This high temperature calcination can degrade certain properties of zeolites. While not wishing to be bound by theory, this could potentially include dehydroxylation of silanol groups at the surface of the zeolite, where these groups are necessary for a high degree of attachment of silating agents. These silating agents can provide a bonding link between the zeolite and the membrane polymer phase. Without this link, gas may bypass the zeolite particles, diminishing separation selectivity. Other linking methods via surface silanol groups are also possible, such as through reactive groups in the polymer itself. Again, a decrease of these silanol groups would negatively impact that linking.
Another factor which could decrease zeolite effectiveness is residual amorphous siliceous material at the surface of the zeolite which could block surface sites and/or diminish diffusion of gases through the zeolite. Calcining the zeolite to remove the organic template prior to implementing procedures designed to remove amorphous material could anchor the amorphous material at the zeolite surface, making it difficult to remove and leading to poorer membrane performance. Blocking of surface sites could also lead to a diminishing of the surface charge (Zeta-potential) of the zeolite, making the zeolite particles easier to agglomerate during membrane formation which could also lead to poorer membrane performance.
Thus, there is a need to produce mixed matrix membranes with higher permeability and selectivity for a given loading of molecular sieves. The present invention addresses this need by overcoming some of the above described shortcomings of conventional mixed matrix membranes and in their manufacture.